1. Field of the Invention
The present invention relates to a method of fabricating a polysilicon film, and more particularly, to a method of fabricating a polysilicon film by an excimer laser crystallization (ELC) process.
2. Description of the Prior Art
The progress of science and technology has led to small, effective, and portable intelligent information products becoming a part of our lives. Display devices play an important role because all intelligent information products, such as mobile phones, personal digital assistants (PDAs), or notebooks, need display devices to be a communication interface. However, the fully developed amorphous silicon thin film transistor liquid crystal display (a-Si TFT LCD) devices, which are restricted in their carrier transfer rate, have difficulty in meeting the requirements of being thin, being power saving, and having high resolution. Therefore, the a-Si TFT LCD devices are gradually replaced by low temperature polysilicon (LTPS) thin film transistor liquid crystal display (TFT LCD) devices.
In the liquid crystal display devices, since a normal glass substrate can only work at a temperature below 600° C., fabricating a polysilicon film directly under a high temperature will make the glass substrate twisted. Thus, in a conventional method of fabricating a polysilicon thin film transistor, an expensive quartz substrate is needed and only a small size liquid crystal display panel can be made. Recently, a method of forming a low temperature polysilicon film by crystallizing an amorphous silicon film is gradually developed. Among the methods of forming polysilicon film, the excimer laser crystallization process is the major focus.
Furthermore, a plurality of low temperature polysilicon thin film transistors arranged in a matrix form is disposed on the LCD panel for driving the pixel electrodes in the LCD panel to display images. Thus, the polysilicon film normally comprises a plurality of polysilicon islands. Each of the polysilicon islands functions as an active area to form a source, a drain, and a channel region of each low temperature polysilicon thin film transistor.
For clarity, only one polysilicon island is illustrated in following diagrams to show a conventional method of fabricating a polysilicon film by an excimer laser crystallization process though the polysilicon film comprises a plurality of polysilicon islands in fact. Referring FIG. 1 to FIG. 4, which are schematic diagrams of a method of fabricating a polysilicon film by an excimer laser crystallization process according to prior art. As shown in FIG. 1, a display panel 10 comprising a substrate 12 is first provided. Then, a sputtering process is performed to form a metal layer on the substrate 12. A first photo-etching process is followed to pattern the metal layer and form an alignment mark 14 on the surface of the substrate 12. In the preferred embodiment of the present invention, the substrate is a glass substrate. The alignment mark 14 comprises at least an uneven structure disposed in a peripheral region of the substrate 12 which is not used for circuit layout. Therefore, the alignment mark 14 can be recognized by working machines clearly after performing a plurality of deposition processes.
A plurality of lithography processes are typically used in the fabricating process of the polysilicon film and the following manufacturing process of the display panel. If there is any bias present in the lithography processes, the device reliability is deteriorated and the device is malfunctioned if some serious defects occur. Therefore, in order to improve the alignment ability, the working machines have to align themselves according to the alignment mark 14 before performing any operations, especially the lithography process, reducing the possibility of generating defect due to misalignment.
As shown in FIG. 2, a buffer layer 16 and an amorphous silicon film 18 are formed on the surface of the substrate in turn. The amorphous silicon film 18 is defined with a first region 20 and second region 30. As shown in FIG. 3, a patterned mask layer 22 is formed on the second region 30. The mask layer 22 may have a single layer structure composed of a metal layer or a silicon nitride layer, or a multi-layer structure composed of a combination of materials mentioned above. The mask layer 22 can utilize the metal layer to increase the reflection rate of the second region 30 to reduce the heat absorption of the amorphous silicon film 18 or use the high heat dissipation of the silicon nitride layer to make the amorphous silicon film 18 with the mask layer 22 covering thereon generate grains first. Generally speaking, the formation of the mask layer 22 is used for making the amorphous silicon film 18 in the second region 30, which is covered by the mask layer 22, become partially melted and the amorphous silicon film 18 in the first region 20, which is not covered by the mask layer 22, become completely melted. Thus, when the melted amorphous silicon film 18 solidifies after the excimer laser irradiation is finished, a nucleation will happen based on the partially melted region since there is a heterogeneous interface between the partially melted region and the completely melted region. Grains are grown laterally from the second region 30, which is partially melted, toward the first region 20, which is completely melted, to form a polysilicon film 24 in the first region 20.
As shown in FIG. 4, a photo-etching process is followed to remove the mask layer 22 and the amorphous silicon film 18 in the second region 30 to form a polysilicon island 24 in the first region 20. After that, latter processes of liquid crystal panel manufacturing are performed while the polysilicon island 24 is used as an active area to form a driving circuit of the display panel 10.
In the aforementioned excimer laser crystallization process, the lithography process is needed for the steps of defining the position of the alignment mark 14, patterning the mask layer 22, and forming the polysilicon island 24. In other words, total three lithography processes are used in fabricating the polysilicon island 24. Although the aforementioned method can control the position of the grain boundary, the fabricating process is complex, leading to more fabricating time and cost. Thus, there is a strong need to simplify the excimer laser crystallization process.